Make ahead: Whisk the yeast, coconut milk, and caster sugar in a large bowl. Wrap with cling film and leave for 40 minutes in a warm place for the yeast to activate.
To the flours and bicarbonate of soda, add the coconut mixture and fizzy water to the consistency of double cream. Whisk and leave for a minimum of 2 hours (I usually leave overnight in a warm place unless in a hurry).
On the day: Once your hopper batter is bubbly and smells a bit like beer, it is ready. Heat a hopper pan or frying pan until it is hot but not smoking. Ladle about half a cup of batter into the pan and, once it starts to bubble, swivel a full 360-degree turn to coat the sides of the pan. Crack an egg into the middle of the pancake if you like.
Cover the pan with a lid or tin foil and steam for 2-3 minutes until the sides of the hopper start to separate from the pan (and the egg is cooked, if using). With a wooden spatula, carefully remove the hopper and serve hot.
Every time you use the hopper mix, give it a good stir with a whisk as the batter will separate. You can freeze the mix freshly made, but if not, it only lasts a day or so.